Rats were kindled by repeated injections of pentylenetetrazole (PTZ; 37.5 mg/kg; i.p.) in the presence or absence of the opioid receptor antagonist naloxone. Naloxone (10 mg/kg; i.p.) applied 30 min before each PTZ application had no major effect on the seizure development, although a slight decrease in the seizure expression of fully kindled animals could be observed. In the kindled animals, a pronounced but transient increase in c-fos mRNA level was observed in several brain areas after the injection of PTZ. The magnitude of c-fos induction was related to the seizure stage reached. Detectable c-fos mRNA levels in the cortex were observed in rats showing stage four seizures, whereas the expression of c-fos in the hippocampus required stage five kindled seizures. The induction of c-fos expression in the hippocampus of stage five kindled rats but not in other brain areas was prevented by treatment of naloxone prior to each PTZ application. In contrast, a single injection of naloxone to kindled rats was not sufficient to prevent c-fos mRNA expression in the hippocampus. In addition, a single PTZ application (at the higher dose of 45 mg/kg) to rats that were not kindled also caused c-fos expression in several brain regions, but this was not influenced by naloxone. Assuming that c-fos expression reflects neuronal activity our findings suggest a functional role of endogenous opioid peptides in the development of kindling-induced neuronal excitation in the hippocampus. In addition, the excitation of the hippocampus does not appear to be involved in the seizure activity but may be responsible for the impairment of learning in PTZ-kindled rats which can be prevented by pretreatment with naloxone [A. Becker, G. Grecksch, M. Brosz, Naloxone ameliorates the learning deficit induced by pentylenetetrazole kindling in rats, Eur. J. Neurosci. 6 (1994) 1512-1515].
Copyright 1998 Elsevier Science B. V.